(1966) "At the Brink of Hell"
Last episode's events attract a Twonoser fleet to the secret base on Arctis. But when the Twonosers fail to defeat the Terrans, their Andromedan overlords, the "Masters of the Island", send a space whale to destroy the entire system. I'm not sure what Doc Savage is doing on the cover, but the image seems to reference a subplot in which a Terran agent is blown into an overlapping universe at a different "energy level"